In the process for making flash-spun fibers, as disclosed in U.S. Pat. No. 3,081,519 to Blades et al. assigned to E.I. du Pont de Nemours and Company, Wilmington, Del. (hereafter DuPont), a solution of fiber-forming polymer in a liquid spin agent that is not a solvent for the polymer below the liquid's normal boiling point, is maintained at a temperature above the normal boiling point of the liquid and at autogenous pressure or greater, and is then spun into a zone of lower temperature and substantially lower pressure to generate plexifilamentary film-fibril strands. As disclosed in U.S. Pat. No. 3,227,794 to Anderson et al. (assigned to DuPont), the flash-spinning process requires a spin agent that: (1) is a non-solvent to the polymer below the spin agent's normal boiling point; (2) forms a solution with the polymer at high pressure; (3) forms a desired two-phase dispersion with the polymer when the solution pressure is reduced slightly in a letdown chamber; and (4) flash vaporizes when released from the letdown chamber into a zone of substantially lower pressure.
Commercial spunbonded products made from polyethylene plexifilamentary film-fibril strands have been produced by flash-spinning a spin fluid comprised of polyethylene in a trichlorofluoromethane spin agent. Unfortunately, trichlorofluoromethane is considered to be a stratospheric ozone depletion chemical, and therefore, there is a need for alternative spin agents for use in the flash-spinning process.
The Ozone Depletion Potential (“ODP”) for a compound is a relative measure of the expected impact of the compound on the depletion of stratospheric ozone when the compound is released as a gas into the atmosphere as compared to the impact expected from the release of the same mass of trichlorofluoromethane gas. ODP values are used to compare the relative impacts of the release of different gases upon the Earth's ozone layer. The ODP values are generally calculated by methods like those described in Chapter 13 of “Scientific Assessment of Ozone Depletion: 1994,” Report No. 37 of the World Meteorological Organization's Global Ozone Research and Monitoring Project.
The Global Warming Potential (“GWP”) for a compound is a relative measure of the expected impact on the greenhouse warming of the Earth's atmosphere when the compound is released as a gas into the atmosphere as compared to the impact expected from the release of the same mass of carbon dioxide gas into the atmosphere. GWP is dependent on the degree of absorbance of longwave radiation (infared) by the compound, and the expected lifetime of the compound in the atmosphere. GWP values are generally calculated by methods like those described in Chapter 13 of “Scientific Assessment of Ozone Depletion: 1994,” Report No. 37 of the World Meteorological Organization's Global Ozone Research and Monitoring Project.
In the flash-spinning process, the liquid spin agent is vaporized after passing through the spin orifice. The gaseous spin agent is conventionally collected, condensed, purified, and then recycled into the spin agent feed stream for the flash-spinning process. However, it is likely that a certain amount of the gaseous spin agent will escape to the atmosphere at some point. Accordingly, there is a need to find a spin agent that works well in the flash-spinning process that also has a very low ODP, a very low GWP, a boiling point of less than 100° C., and either no flash point or a flash point greater than 0° C.
U.S. Pat. No. 5,032,326 to Shin (assigned to DuPont) discloses an alternative flash-spinning spin agent, namely, dichloromethane (also referred to as methylene chloride) and a halocarbon co-spin agent having a boiling point between 0° C. and −50° C.
Published Japanese Application JP52633 10-A (published Oct. 12, 1993) discloses that three-dimensional flash-spun fibers made from polymer dissolved in mixtures of spin agents where the major component of the spin agent mixture is selected from the group consisting of dichloromethane, dichloroethylene, and bromochloromethane, and the minor component of the spin agent mixture is selected from the group consisting of dodecafluoropentane, decafluoropentane, and tetradecafluorohexane.
U.S. Pat. No. 5,672,307 (assigned to DuPont) discloses a process for flash-spinning plexifilamentary film-fibril strands from polyolefin polymer dissolved in mixtures of spin agents where the major component of the spin agent mixture is selected from the group consisting of dichloromethane and dichloroethylene, and the minor component of the spin agent mixture is selected from the group consisting of hydrofluoroethers and cyclic perfluorinated hydrocarbons, wherein the minor component of the spin agent has 3 to 7 carbon atoms and an atmospheric boiling point between 15° C. and 100° C.
U.S. Pat. No. 5,874,036 (assigned to DuPont) discloses a process for flash-spinning plexifilamentary film-fibril strands from polyolefin polymer dissolved in mixtures of spin agents where the major component of the spin agent mixture is selected from the group consisting of dichloromethane and dichloroethylene, and the minor component of the spin agent mixture is a cyclic partially fluorinated hydrocarbon having 4 to 7 carbon atoms and an atmospheric boiling point between 15° C. and 100° C.
The co-spin agents disclosed in the four patent publications discussed above do not exhibit the desired combination of having a very low ODP, a very low GWP, a boiling point of less than 100° C., and either no flash point or a flash point greater than 0° C. Accordingly, there is a need for an alternative co-spin agent for use in the flash-spinning process, which co-spin agent has a very low ODP, a very low GWP, a boiling point of less than 100° C., and either no flash point or a flash point greater than 0° C.